Method for hard-chrome plating large metallic surfaces

ABSTRACT

A method of hard-chromium plating elongated bodies of large surface area wherein the body is advanced in stages through the chromium plating bath relative to the anode, and chromium plating is carried out during the transverse in which the substrate is stationary. A pair of equipotential screens is provided in axially spaced relationship at each end of the plating zone, at least one of the screens being positioned upon the previously coated portion of the substrate at a location in which the prior plating has reached its maximum thickness.

Unite States atet [72] Inventors Antoaneta M. Draghicescu;

Aurel C. Radoi, both of Bucharest, Romania [21] Appl. No. 813,828 [22]Filed Apr. 7, 1969 [45] Patented Oct. 26, 1971 [73] Assignee Institutulde Cercetari Technologice Pentru Constructii de Masini [54] METHOD FORHARD-CHROME PLATING LARGE METALLIC SURFACES 6 Claims, 5 Drawing Figs.

[52] US. Cl 204/25, 204/28, 204/211 [51] Int. Cl C23b 5/56, C23b 5/58,BOlk 3/00 [50] Field of Search 204/DIG. 7, 25, 23,15, 28, 211

[56] References Cited UNITED STATES PATENTS 1,794,973 3/1931 McBride204/28 2,232,019 2/1941 Beckwith 204/28 2,540,175 2/1951 Rosenquist..204/9 3,415,723 12/1968 Bediet al.. 204/15 3,477,920 11/1969 Bedi 204/15FOREIGN PATENTS 734,414 5/1966 Canada Primary Examiner-T. Tung AssistantExaminerT. Tufariello Attorney-Karl F. Ross Hard-Chrome Plating BefhPATENTEDnm 26 ml SHEET 1 [1F 4 mEQ W 5S 1 g Attorney PATENTEDum 26 IanSHEET 2 [IF 4 Anfoanef'a M. Draghicescu Aurel C. Radoi Invenfors.

K rl 6R0 Attorney PAIENTEBncI 26 men 3.616287 sum 3 BF 4 x/CJ AnfoanefaM. Draghicescu Aurel C. Radol' lnvenfors.

' Attorney METHOD FOR HARD-CHROME PLATING LARGE METALLIC SURFACES Ourpresent invention relates to a method of hard-chromium platinglarge-surface elongated bodies or substrates and especially elongatedshafts, rods, bars and the like which may be used inmechanical-engineering systems, in petroleum engineering and in navalengineering, including (but not exclusively) pistons and other rams,elevator support tubes and shafts, turbine shafts, the power shafts forthe screws of ships and like elongated metallic bodies which may have alength of the order of tens of meters.

Various methods have been provided heretofore for the hardchromiumplating of large-surface bodies, such methods generally requiringplating tanks and installations of correspondingly large dimensions andhigh-intensity (high-amplitude) electroplating current sources.

The hard-chromium plating of a body requires a certain electricalcurrent density to obtain the desired degree and quality of the coatingand, as the surface area of the body to be placed increases, acorresponding increase in the current supply capacity of the powersource is necessary.

Electroplating processes using high-intensity (high-amplitude)electroplating currents, large quantities of electrolyte and large tanksand other installations have heretofore created control and supervisorydifficulties, thereby preventing the obtention of uniform hard-chromiumplating over the entire surface of a large body, and rendering theplating of large bodies nonreproducible. In fact, the hard-chromiumplating of elongated bodies of the character described has heretoforebeen troublesome and difficult.

It is the principal object of the present invention to provide animproved method of coating large-surface bodies by hardchromiumelectroplating.

Another object of our invention is the provision of an improvedapparatus for the hard-chromium plating of large bodies for the purposesand of the character described.

Another object of this invention is to provide a method of and anapparatus for the hard-chromium plating of elongated bodies, e.g., of alength of the order of tens of meters, which makes use of currentsources of unexpectedly low amplitude capacity, uses relatively smallamounts of electrolyte, is readily controlled and supervised, and givesrise to uniform reproducible coatings over the entire length ofthe body.

These objects and others which will become apparent hereinafter areattained, in accordance with the present invention, by a method ofhard-chromium plating elongated bodies whereby the body is advanced instages through an electroplating bath relatively to the counterelectrodeor anode which is juxtaposed with only a portion or increment of thelength of the body to be plated flanking this anode and maintained atthe potential of the cathode or substrate, there is provided a pair ofequipotential screens which surround the body and define the platingzone for each increment or plating stage.

After an initial plating of the substrate or workpiece, at least onesuch screen is provided (at the junction between a prior platingincrement and the zone to be subsequently plated) on the prior platingin a region in which the latter is of its maximum thickness. Theequipotential screens, according to this invention, have a conductiveperipheral outline at the same potential as the cathode or workpiece andgeometrically similar to the periphery thereof, so that when the platedbody is of circular section, the screens are likewise of circularoutline.

We have found that a pair of conductive shields, spaced outwardly fromthe cathodic workpiece or substrate by a holder or nonconductivematerial, cause the plating in the immediate regions of these screensand within the plating zone to taper off in a remarkably uniform manner.Thus, when an equipotential screen or shield is provided at the end of apreceding plating zone, the coating in this region tapers from itsmaximum thickness to zero at the equipotential screen, the coatingterminating in a plane of the edge of the conductive screenperpendicular to the major dimension or direction of elongation of theworkpiece body.

Thus, by using two such screens, one of which is positioned so that itsplane of plating termination lies precisely at the end of the precedingtaper at the thickest coating in the prior plating zone, it is possibleto overlap the taper coating to obtain a hard-chromium plating ofconstant thickness over the length of the substrate, in spite of thefact that the latter is electroplated in increments or stages.

According to another aspect of this invention, the apparatus forhard-chromium plating elongated metallic bodies of tens of meters inlength may include a tank for the chromium-plating bath which has alength corresponding to a small fraction of the length of the workpieceand is provided with a seal at its opposite ends so that the workpiecemay be advanced in stages through this tank.

The anode, according to this invention, is a sleeve surrounding theworkpiece over the major part of the length increment within the tankwith all around uniform clearance and has a configuration geometricallysimilar to the cross section of the workpiece and the equipotentialscreens mentioned earlier. The latter are provided with means enablingthem to be shifted along the workpiece to be detached or replacedthereon axially outwardly of the anode, the internal diameter of thelatter preferably being greater than the diameter of the metallicscreens or shields.

The apparatus may also be provided with polarity-reversing means wherebythe workpiece may be connected to the positive terminal of the sourcewhile the counterelectrode is connected to the negative terminal for abrief anodization of the workpiece prior to hard-chromium plating, theanodization serving to increase the strength of the bond between thechromium coating and the substrate.

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is an elevational view, partly in longitudinal cross section in avertical plane through a chromium-plating tank according to thisinvention;

FIG. 2 is a longitudinal partial section in a vertical plane through anequipotential screen according to this invention;

FIG. 2A is a section along line IIA-lIA of FIG. 2;

FIG. 3 is an elevational diagram showing the positions of the anode andscreen during the initial plating operation; and

FIG. 4 is a diagram similar to FIG. 3 showing the positions of thescreens during the subsequent plating operation.

The system as illustrated in FIG. 1 is an apparatus for thehard-chromium plating of an elongated body 1 of circular cross section,whose exterior is rough from a preliminary degreasing and/or descalingoperation, which extends through the chromium plating tank 2 throughopenings 2a provided in the end walls 2b and 2c, thereof. These openingsare provided with a gland-type seal as represented at 3 to preventescape of the chromium-plating electrolyte M in which the region la ofthe body 1 within the bath 2d is immersed.

Coaxial with the body la and extending over the major part of the lengththereof within the tank 2 is an anode 4 of geometrically similarconfiguration (cylindrical when the body I is a shaft of circular crosssection) composed of an antimony-lead or a lead-tin-silver alloy andperforated with holes 4a. The anode 4 is spaced by to mm. from theworkpiece. The surface of the anode relative to the cathodic workpieceis in a ratio therewith ranging between 2:1 and 1:1.

Outwardly of the anode 4 is provided a pair of shields 5a and 5b whichare best shown in FIG. 2 and may be spaced from the anode 4 by adistance of 40 to 60 mm.

The holes or apertures 40 in the anode 4 are provided in longitudinallyextending arrays with successive arrays about the periphery of the anodebeing staggered by approximately half the distance between the holes ofeach array.

As can be seen in FIGS. 2 and 2A, each shield 5a (or 5b as seen in FIG.3) comprises a pair of semicircular ring members 6 held together byscrews 6a through lugs provided on the ring halves (FIG. 2A) andcomposed of electrically insulating material. A metallic band 8 has acylindrical sleeve 80 fixed to the outer periphery of the ring halves 6by screws 7 and axially overhangs the ring at 822 while being providedwith an inwardly turned flange 8c, the inner periphery 8d of whichspacedly surrounds the workpiece 1 when the ring is placed thereon.

As shown in FIG. 1, a reversible power supply is provided for thehard-chromium plating apparatus. This power supply comprises the usualline source 10 of alternating current which energizes a rectifierarrangement 12 via a current-controlling component 11 such as anautotransformer. A reversing switch 13 of the double-poledIdouble-throwtype is ganged with a single-pole/single-throw switch 14 so that, whenthe switch is in the position illustrated, the positive terminal 120 ofthe rectifier can be connected to the workpiece 1 while the negativeterminal 12b is connected to the anode 4. In this position switch 14 isopened and the shields a and 5b are deenergized.

When the switch arrangement l3, 14 is reversed, the positive tenninal12a is connected to the anode 4 and the negative terminal l2b to theworkpiece 1 while switch 14 closes to apply the cathode potential to theshields 5a and 5b. The operation of the device will be described ingreater detail in connection with a specific example.

EXAMPLE Using the apparatus of FIGS. 1-4 a shaft of circular crosssection with a length of 30 m. was introduced into the chromium-platingtank 2. The anode 4 was provided with holes 4a with diameter of 5 to mm.and an interhole spacing of about 10 to 20 mm. The rows of holes werecircumferentially spaced by about 10 to 30 mm. The surface area of theanode 4 confronting the workpiece had a ratio to the workpiece area ofabout l.5:l and the anode is spaced by 125 mm. from the workpiece. Theshields 5a and 5b were mounted at a distance of about 40 to 60 mm. fromthe anode and positioned as described below.

For the initial chromium-plating operating only a single shield 5b,positioned at a distance d about 50 mm. from the anode 4 was used asshown in FIG. 3. A conventional hardchrornium plating electrolytecontaining 200 to 250 g./l. chromium trioxide (Cro and 2 to 2.5 g./l.sulfate ion (S0 and constituted as described in the Encyclopedia ofElectrochemistry, pages 201 ff., Reinhold Publishing Corp., N.Y., 1964,was used at a temperature of the plating bath of 55 C. An initialanodization (with the workpiece 1 connected to the positive terminal12a) for a period of 90 seconds with increas ing current density 12 to40 a./dm.") is carried out to prepare the workpiece for plating. Theelectroplating is carried out at 40 to 60 a./dm. (FIG. 3) to form acoating C of the desired thickness. This coating has a stretch C l ofuniform thickness terminating in a tapering portion C of diminishingthickness terminating in a plane P which is the plane of the flange 8cof the shield 5b. The axial length of the tapering portion C is alsorepresented by the distance d. The junction of the tapered portion andthe uniform-thickness portion is represented at C For the next stage orincrement, the workpiece l is advanced through the tank by a distanceequal approximately to the axial length of the anode 4 and the distanced with the portion of the workpiece emerging from the tank beingsuggested to washing, e.g., via a spray nozzle 15 and drying asrepresented by the heating coil 16.

As shown in FIG. 4, shield 5b is mounted so that its flange 8a lies inthe plane P of the junction between the tapered portion C and thenconducts uniform-thickness portion C while a section shield or screen Sois placed downstream at the other end of the plating zone at a distanced of 40 to 60 mm. from the anode 4 and in the case of the presentexample, at about 50 mm. from the end of this anode. Again plating iscarried out under the indicated conditions, whereupon the shields areshifted to the next increment, etc.

As shown in broken line in FIG. 4, a tapered plating is formed over theprevious tapered portion so that the junction between the two incrementshas the same thickness as the plating between these unctions. In eachincrement, a brief the steps of:

a. advancing said surface in increments through a plating bath tojuxtapose successive portions of said surface with a counterelectrode;

b. positioning on said surface upstream of said counterelectrode in thedirection of advance of said surface and at a predetermined spacing fromsaid counterelectrode, a conductive equipotential shield substantiallyat the electrical potential of said surface;

c. cathodically connecting said surface and anodically connecting saidcounterelectrode to electrodeposit a metal from said bath onto saidportion of said surface juxtaposed with said counterelectrode with thedeposited coating tapering off between said counterelectrode and saidshield;

d. positioning a successive portion of said surface in juxtapositionwith said counterelectrode while locating a shield on the previouslyformed coating substantially at the junction of the tapered portionthereof with the remainder of the previously formed coating and at adistance from the counterelectrode corresponding to the distance betweenthe counterelectrode and the shield during the electroplating of thepreceding portion, positioning a further conductive equipotential shieldupstream of said counterelectrode, and then electroplating saidsuccessive portion of said surface between the shields by cathodicallyconnecting said surface and anodically connecting said counterelectrode;and

e. repeating step (d) until said surface is completely coated to auniform thickness.

2. The method defined in claim 1 wherein said portions of said surfaceare plated with a hard-chromium electroplate at a current density of 40to 60 a./drn. said counterelectrode has a surface area having a ratio tothe confronting surface area of said portions ranging between 2:] and1:1 and spaced from to mm. from said metallic surface, and said bath isa hard-chromium plating bath containing chromium trioxide and sulfateion.

3. The method defined in claim 2 wherein said metallic sur face is thesurface of an elongated body, said body being ad vanced axially throughsaid bath in said increments, said shields being positioned around saidbody, said anode surrounding said body between the shields of step d).

4. The method defined in claim 3 wherein said counterelectrode isperforated and is provided with staggered rows of holes having adiameter of 5 to 10 mm., said counterelectrode being composed of alead-antimony alloy or lesd-tin-silver alloy.

5. The method defined in claim 1, further comprising the step of brieflyanodically connecting said surface and cathodically connecting saidcounterelectrode to anodize each portion of the surface juxtaposed withsaid counterelectrode prior to the electrode deposition of metal fromsaid bath onto each portion of said surface.

6. The method defined in claim 5 wherein anodization is carried out fora period of about 90 seconds at a current density increasing graduallyfrom 12 to 40 a./dm.*.

2. The method defined in claim 1 wherein said portions of said surfaceare plated with a hard-chromium electroplate at a current density of 40to 60 a./dm.2, said counterelectrode has a surface area having a ratioto the confronting surface area of said portions ranging between 2:1 and1:1 and spaced from 100 to 150 mm. from said metallic surface, and saidbath is a hard-chromium plating bath containing chromium trioxide andsulfate ion.
 3. The method defined in claim 2 wherein said metallicsurface is the surface of an elongated body, said body being advancedaxially through said bath in said increments, said shields beingpositioned around said body, said anode surrounding said body betweenthe shields of step (d).
 4. The method defined in claim 3 wherein saidcounterelectrode is perforated and is provided with staggered rows ofholes having a diameter of 5 to 10 mm., said counterelectrode beingcomposed of a lead-antimony alloy or lead-tin-silver alloy.
 5. Themethod defined in claim 1, further comprising the step of brieflyanodically connecting said surface and cathodically connecting saidcounterelectrode to anodize each portion of the surface juxtaposed withsaid counterelectrode prior to the electrode deposition of metal fromsaid bath onto each portion of said surface.
 6. The method defined inclaim 5 wherein anodization is carried out for a period of about 90seconds at a current density increasing gradually from 12 to 40 a./dm.2.